Flywheel hybrids offer new efficiency potential

  • 08-Sep-2014 09:11 EDT
Torotrak09-14 Jon Hilton 2.jpg

Jon Hilton through the flywheel: He claims that Torotrak's research has disproved the theory that hybrids are only effective in urban driving. (For more images, click the arrow at the upper right of this box.)

A decade ago, the current global requirements for future new-car CO2 emissions—USA 2025, 93g/km; EU 2020 95g/km (possibly 70g/km by 2025); China 2020, 117g/km; and Japan 2020, 105g/km—would have seemed like numbers from fantasyland. What's more, the challenges that these figures herald are compounded by the “get real” demands to move away from “official” but often unrepresentative published fuel consumption figures.

The Worldwide harmonized Light Vehicle Test Procedure (WLTP) scheduled for introduction in 2017, is aimed at providing car buyers with fuel consumption figures that will allow them to budget with confidence when choosing a vehicle. The WLTP is set to change OEMs’ approach to achieving emissions/fuel consumption compliance. “Cycle beating” strategies will be out, reality will be in, and new test procedures will be a must.

Jon Hilton, Product Development Director of Torotrak, a U.K-based specialist powertrain technology company, believes that keeping pace with the new developments in legislation will require not just incremental advances but a step-change in real-world vehicle efficiency performance to deliver required lower CO2 and other pollutant (such as particulates) emission levels.

The challenges are most likely to be met by redefining the role of the ICE (internal combustion engine), he said. “Today’s mild hybrids may typically consist of a 120-kW combustion engine with a 15-kW hybrid power source because this is the most affordable combination, but reaching future targets will require an opposite approach, with something like a 30-kW engine and a 100-kW electric drive. But that does not mean demoting the ICE to become a mere range extender.”

The weight, cost and packaging challenges of producing such an arrangement with electric hybrid power will provide an opportunity for a mechanical kinetic energy recovery system based on high speed flywheel technology, noted Hilton who has been involved in some 20 projects that have used the technology. Torotrak’s Flybrid energy recovery system uses a mechanically-driven flywheel to capture kinetic energy during braking and efficiently return it to the wheels (

Enabler for engine downspeeding

Flywheel hybrids are typically around a third of the weight and a quarter of the cost of an equivalent electric system thanks to their use of familiar and easily manufactured components, and to their inherent power density, he explained.

Hilton has identified the synergies that can exist between a large flywheel energy source and a combustion engine. To explore how they could be harnessed, he is working with several European vehicle manufacturers. “Fundamentally, you use the flywheel for acceleration and the engine for cruising," he said. "The flywheel can also power the car, start the engine or supplement engine power for additional performance. To ensure that the expected acceleration is consistently available, the control system uses the engine to efficiently spin-up the flywheel when there is insufficient regenerated energy.”

He claims that Torotrak’s research in this field has disproved the theory that hybrids are only effective in urban driving. Extensive field trials with data logging covering many types of driver, vehicle and route, had enabled accurate simulation to be achieved with the data showing a clear benefit with flywheel hybrids, even when driving on the open road, he said.

Under steady cruise conditions, when the engine is lightly loaded, BSFC (Brake Specific Fuel Consumption) is rarely at its optimum value. While charging the flywheel, the engine is placed under slightly greater load and therefore operates more efficiently, Hilton explained. Once the flywheel is energized, the engine is switched off and the stored kinetic energy released to power the vehicle, then the cycle is repeated.

This ‘boost and cruise’ approach has contributed to Volvo’s Flybrid demonstrator achieving a 25% fuel efficiency improvement in real world driving, compared to an equivalent pure ICE powertrain. Hilton instanced one case, using a 2.0-L sedan on a 24-km (15-mi) cross-country route, when the predicted BSFC improved from 470 g/kW·h to 280 g/kW·h when charging the flywheel.

Simulation of a 1000-kg (2200-lb) B-segment car with a 0.9-L 30-kW (40-hp) engine mated to a mechanical flywheel system showed 58 g/km CO2, equivalent to 2.5L/100 km fuel consumption, on the U.S. FTP75 drive cycle. This would satisfy the proposed 2030 EU targets and 2035 U.S. targets without requiring the cost and risk of any new technology, Hilton claimed, adding that such a car could be built for production today using the Flybrid technology.

Potential collateral benefits of mechanical flywheel hybrids will bring various additional advantages including making engine downspeeding easier to achieve, he said. This would facilitate significant further improvements; notably an engine running at half its original speed suffers only a quarter of the original friction losses.

But downspeeding reduces the energy in the exhaust available to spool-up a turbocharger. However, releasing energy from a flywheel would overcome this turbo lag, providing the necessary in-fill torque without recourse to a probably costly bi-turbo solution.

Torotrak’s Flybrid system, on which R&D began in 2007, is described by Hilton as incorporating advanced flywheel technology. Its advanced carbon composite construction allows the flywheel to spin safely at speeds up to 60,000 rpm. As energy increases with speed squared, so doubling the speed stores four times the energy within the same package. Ironically, using a flywheel material with greater mass, such as steel, would actually reduce the safe operating speed to a level where the stored energy would be lower, he explained.

To meet the safety requirements of the SAE J1240 standard, the minimum burst speed of a steel flywheel must be 2.6 times the maximum operating speed. To keep within safe working stresses would limit a steel design similar in size to that of the Torotrak flywheel, to around 20,000 rpm.

Carbon construction has a fundamental safety advantage over steel, he noted. Because it is filament wound, any delamination generating long, lightweight fibers that can be easily contained, and which dissipate energy more effectively.

The other key element in Torotrak’s mechanical hybrid technology is the clutched flywheel transmission (CFT) that integrates the flywheel into the powertrain while allowing flywheel speed to remain independent of engine speed. So the flywheel can increase in speed through energy transfer under braking without influencing engine speed. Energy can also be released to the vehicle during the (constant-speed) cruise as well as during acceleration.

Next phase: Transmission integration

After being developed for Flybrid Kinetic Energy Recovery System racing projects, the CFT also offers what Hilton termed “exceptional” response time. This allows the flywheel to be rapidly charged from even a brief touch of the brake pedal. Energy transfer rates can be very high without the degradation of storage capacity suffered by batteries that are subjected to rapid charging.

The powertrain architecture also provides virtually instant torque for immediate accelerator response including rapid step-off, a strong point of pure electric power systems.

All advanced technology automotive systems, however impressive their potential, must be totally cost effective—a major aspect of hybrid credibility in both the OEM’s and the end-user’s analysis. A regular electric hybrid, with its use of a high voltage system and battery pack and controls, may add some 20% to the cost of an equivalent ICE only model. The Flybrid system applied to a high volume vehicle would be “significantly” less, believes Hilton.

This is particularly significant, as Torotrak’s planned next development phase is to see its system integrated into a gearbox, giving it a compact architecture that could eventually lead to high volume passenger car applications. Sharing components such as casings, and cooling and lubrication systems, there would be a substantial reduction in size (~30% fewer parts), weight (also ~30%) and unit cost.

Hilton is very bullish about flywheel technology’s future: “Engine downsizing potentially releases enough space to integrate the flywheel system within the existing package of today’s typical powertrains," he argues. "That makes it much easier for OEMs to introduce the technology.”

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